Turtles (chelonians) exhibit the remarkable ability to survive long periods without breathing. For example during diving in aquatic turtles, breatholds for 1 to 2 hours have been observed. The organ that plays a critical role in the turtles remarkable breatholding ability during diving is the heart. The heart of the turtle is comprised of only 3 chambers (as compared to the 4 chambers of mammals and humans). The unique anatomy results in the potential for the blood returning from the body to either circulate through the lungs or to bypass the lungs. When the blood bypasses the lungs it is referred to as a right-to-left intracardiac shunt. Previous scientific investigations have reported that the amount of blood bypassing the lung is often coupled with ventilatory patterns, with the right-to-left shunt increasing during breatholding. In contrast, during breathing the amount of blood circulating to the lungs increases and the right- to-left shunt is reduced. While a large right-to-left shunt would produce harmful results in mammals and humans, the ability of reptiles to bypass the lungs may play several important roles during breatholding. These include reducing the energy expenditure of the heart, the "metering" of oxygen from the lung (analogous to a scuba tank) and maintaining the balance of fluids between the lung and blood. However, the precise process by which the heart of turtles can accomplish varying levels of blood shunting have been controversial and not well understood. The study proposed by Dr. Hicks will utilize a combination of sophisticated radiological techniques, blood flow measurements and injections of tracer gases to image the heart and determine blood flow patterns through the heart during a variety of physiological conditions. These techniques and the experiments outlined in Dr. Hicks' proposal will determine if the process of blood shunting results either from factors external to the heart (i.e. regulation by the lung) or factors intrinsic to the heart to understanding the evolutionary development of the cardiovascular system in vertebrate animals. His elucidation of the mechanism of blood shunting in reptiles will help to answer the question of what functional role blood shunting plays in this ancient but important group of animals.
